1. Technical Field
The present invention relates to direction finding (DF) systems and more specifically, to DF systems using circular antenna arrays.
2. Prior Art
Most presently used techniques for analysing received signals from circular arrays of antennas equally spaced around an imaginary circle are based on either the spatial Fourier series method or the Butler matrix method. The Butler matrix method is described in a paper by J. Butler and R. Lowe entitled "Beam Forming Matrix Simplifies Design of Electronically Scanned Antennas" in Electronic Design, Apr. 12, 1961, Pages 170-173. The limitations of these techniques are described in a paper entitled "Studies of the Adcock Direction Finder in terms of Phase-mode Excitations Around a Circular Array" by Guy et al in "The Radio and Electronic Engineer Vol.53, No.1, pp33-38, January 1983.
Practical implementations of these techniques are described, by way of example, in GB-A-1593286 and GB-A-2095497.
Both these methods employ a number of approximations in their analysis which limit their accuracy and it would therefore be desirable to provide a method of analysis suitable for implementation with a circular antenna array that disposed of all analytical approximations and provided a potentially output of the bearing of a received signal.
In most typical electromagnetic environments in which DF systems are currently used, there are many signals being received by the antennas. Unless the receivers used are to be tuned to a particular frequency, it is essential that the DF system should be capable of accurately indicating the presence of and discriminating between the bearing directions of a large number of signals of different frequencies covering the whole of a received wide bandwidth.
U.K. Patent Application No. 2076152A (Krupps) shows a direction finding system using a 4 antenna Adcock array which is adapted to be frequency selective. The signals received by each antenna are fed to a Fast Fourier Transform (FFT) processing block which outputs a spectrum of the received signal. Cross spectra of the signals received from opposite antennas are produced and, from these, a bearing calculation is performed and an output display representing the incident angles of signals relative to their frequency is generated. The bearing angles themselves are calculated as the arctan of a ratio of transit times determined from phase differences. There is no discussion of the limitations on the size of the array in order to produce accurate bearing outputs. The bearing calculation technique employed depends on having cross spectra from orthogonal apertures and is limited to the case of 3 antennas arranged at the corners of a right angled isosceles triangle or 4 antennas at the corners of a square. The processing method described is relatively elaborate. Moreover when the number of antennas is reduced from 4 to 3, the antenna system will be asymmetric and therefore an imbalance in the electromagnetic mutual coupling between the elements will be produced. This will give rise to phase measurement errors.
The technique of providing frequency selectivity in direction finding systems has been described in British Patent Specification No. 1392343. This specification discloses the use of orthogonal transformations including the Fast Fourier Transform for generating spectra from received antenna signals in direction finding systems. This specification does not discuss in detail appropriate methods for analysing the spectra to produce the required bearing information.